1. Field of the Invention
The present invention relates to a radio frequency generator for a radio communications system, specifically, to a technique for generating the radio frequency signals required for transmission and reception of data in a digital cellular base station.
2. Description of the Related Art
Generally, a transceiver, using the superheterodyne method, needs a frequency generator which up-converts the frequency of transmitted data signals or down-converts the frequency of received data signals. This frequency generator should generates a stable signal suitable for a predetermined prescribed performance in a radio communication system. Since the frequency of a transceiver used in a digital cellular base station must operate within a transmission band of 869 to 894 MHz and reception band of 824 to 849 MHz, the frequency generator should generate a local oscillator signal which can provide signals within the above specified transmission and reception bands.
The radio frequency generator used in a digital cellular base station provides the transceiver with intermediate frequency local oscillator (IF LO) signals and ultra-high frequency local oscillator (UHF LO) signals using a frequency synthesizer in a phase lock loop (PLL) mode. A method proposed by Qualcomm Co. of the United States is an example of a radio frequency generator using the PLL frequency synthesizing method. The frequency generator of Qualcomm Co. consists of a 65.04 MHz frequency synthesizer, a RX frequency synthesizer [UHF(754 to 779 MHz)], and a TX frequency synthesizer [UHF(799 to 824 MHz)]. This radio frequency generator uses the IF LO signal in the transmission and reception portions of the radio communication system, and uses the UHF LO signal in transmission, and UHF LO signal in reception. With a conventional frequency generator, two UHF LO signals having a frequency difference of 45 MHZ are used on the same board, and a spurious effect between the two UHF LO signals is generated.
The Schneider patent, U.S. Pat. No. 3,324,396, entitled Multiple Conversion Transceiver Utilizing Single Oscillator, discloses a basis concept of utilizing a single radio frequency generator for generating local oscillator frequencies for one or more mixer stages in both transmitting and receiving portions of a transceiver.
Similarly, both the O'Connor and Ertman patents, U.S. Pat. Nos. 3,825,830 and 3,509,462 entitled respectively Offset Oscillator System for Radio Transmitter And Receiver, and Spurious-Free Phase-Locked Continuously Tuned Transceiver System, disclose single local oscillators being utilized for both receiver and transmitter portions of transceiver systems.
The Shimakata patent, U.S. Pat. No. 4,627,099, entitled Communication Apparatus For Transmitting And Receiving Signals On Different Frequency Bands, discloses an arrangement in which a reference oscillator output is connected to a plurality of frequency dividers whose output is connected to respective phase lock loop networks to produce different output frequencies.
The Franke et al. patent, U.S. Pat. No. 4,476,575, entitled Radio Transceiver, discloses a radio transceiver in which a single oscillator output is utilized for generating local oscillator signals for the mixer portions of both transmitting and receiving portions of the transceiver.
The Vaisanen patent, U.S. Pat. No. 5,519,885, entitled Method to Generate Different Frequency Signals In A Digital Radio Telephone, discloses an arrangement for a cellular telephone in which different radio frequency signals are generated for use both in transmission and reception portions of a digital radio telephone.
The following references each disclose features in common with the present invention but do not teach or suggest the specifically recited technique for generating radio frequency signals required for transmission and reception of data in a digital cellular base station.
U.S. Pat. No. 5,319,799 to Morita, entitled Single Oscillation Method For Time-Division Duplex Radio Transceiver And Apparatus Using The Same, U.S. Pat. No. 4,528,522 to Matsuura, entitled FM Transceiver Frequency Synthesizer, U.S. Pat. No. 5,574,986 to Mobach, entitled Telecommunication System, And A First Station, A Second Station, And A Transceiver For Use In Such A System, U.S. Pat. No. 5,610,559 to Dent, entitled Dual Loop Frequency Synthesizer having Fractional Dividers, U.S. Pat. No. 5,355,523 to Ogawa et al., entitled Wireless Transmission/Reception System Including Transmitting Terminal Producing Multiple Frequency Local Oscillation Signals And Receiving Terminal Storing A Local Oscillation Signal, U. S. Pat. No. 5,493,710 to Takahara et al., entitled Communication System Having Oscillation Frequency Calibrating Function, and U.S. Pat. No. 5,493,700 to Hietala et al., entitled Automatic Frequency Control Apparatus.